1. Field of the Invention
The invention relates to a method for operating a resonance measuring system, in particular in the form of a Coriolis mass flowmeter or in the form of a density measuring device, wherein the resonance measuring system has at least one measuring tube with a flowing medium, at least one oscillation generator, at least one oscillation sensor and at least one control and evaluation unit, wherein the measuring tube is excited by the oscillation generator to an oscillation with a predetermined excitation frequency and a first amplitude and the resulting oscillation of the measuring tube is detected by at least one oscillation sensor. Furthermore, the invention relates to a resonance measuring system, in particular in the form of a Coriolis mass flowmeter or in the form of a density measuring device that is operated with the above-mentioned method.
2. Description of Related Art
Resonance measuring systems have been known for a long time, not only in the form of Coriolis mass flowmeters, but also as density measuring devices, inter alia. These resonance measuring systems are in contact with a process wherein the process and the resonance measuring system mutually influence one another. Systems in which information about determining process variables (measurement variables) are encoded and/or systems in which working points are placed on the eigenfrequencies of the measuring system are generally the systems called resonance measuring systems, here. The following designs can be used on all of the systems fitting this definition, insofar as they have a measuring tube with medium flowing through it or able to be flowing through it. In the following, resonance measuring systems are shown using Coriolis mass flowmeters as an example, which is not to be seen as being limiting.
Coriolis mass flowmeters are used especially in industrial process measurement, where mass flow has to be determined with high accuracy. The functionality of Coriolis mass flowmeters is based on at least one measuring tube—oscillation element—with medium flowing through it being excited to oscillation with an oscillation generator, wherein the Coriolis inertial force caused by the medium having mass reacts on the walls of the measuring tube due to two orthogonal velocities—the flow and the measuring tube—. This reaction of the medium on the measuring tube leads to a change of the measuring tube oscillation compared to the oscillation state of the measuring tube when there is no flow. By detecting these particularities of the oscillation of the measuring tube with flow—phase differences between measuring tube sections that oscillate in phase when there is no flow in the measuring tube—, the mass flow through the measuring tube can be determined with a high accuracy. Accuracies of about 0.04% of the measured value can be achieved in homogeneous media with high-quality Coriolis mass flowmeters, which is why Coriolis mass flowmeters are often used in custody transfer.
If it is said that the measuring tube is excited with a predetermined excitation frequency and first amplitude, then the predetermined excitation frequency is usually meant as the eigenfrequency of a predetermined or desired eigenform, in which the measuring tube is to oscillate. The excitation frequency is then always corrected by a control and quasi given, should the eigenfrequency corresponding to the predetermined eigenform change in terms of time.
High accuracy and reliability featured in Coriolis mass flowmeters in single-phase flow operation—i.e., during flow of a physically homogenous medium—cannot be readily maintained and achieved in multi-phase flow, particular measures have to be taken here in order to simply acknowledge multi-phase flow; the present invention deals with the detection of multi-phase flow in Coriolis mass flowmeters.
A multi-phase flow is, in general, a flow that is made up of at least two phases with different physical characteristics. The phases, here, can be either of the same or of different materials. Homogenous and spatially limited sections are denoted as phases. The following are examples of such liquid-solid flow, gas-liquid flow, gas-solid flow, water-vapor flow or water-air flow.
It is known that in applications with multi-phase flow, substantial measurement inaccuracies occur, so that it is of great interest that the presence of multi-phase flow can be reliably detected.